FIG. 1 illustrates a typical power transfer curve for a signal amplifier, showing output power Po as a function of input power Pi. The amplifier has a linear operating region, in which an increase in input power causes a corresponding proportional increase in output power. Saturation is reached where the amplifier is no longer able to generate an increasing output as the input continues to increase. Shortly before saturation, the gradient of the transfer curve may already start to reduce from the gradient in the linear region, but this can be considered to remain part of the “linear region”, linearity representing a fixed gradient within a particular tolerance dependent on the application of the amplifier. As the input power increases above saturation, output power may eventually start to decrease as certain secondary effects, specific to the amplifier, begin to arise.
Conventionally, it has been desirable to operate the amplifier at a point on the power transfer curve as close as possible to saturation, since this is associated with maximum signal output power. The amplifier is therefore at its most efficient, in terms of DC power, close to saturation. The operating point of the amplifier can be controlled by controlling the input signal power, potentially using forms of automatic leveling. However, signal distortion is increased at higher power outputs because of non-linearity in the amplifier gain.
Consequently, amplifier configurations typically involve a trade-off between DC efficiency and signal distortion levels. Usually, a compromise is achieved, at which the amplifier is operated below saturation by a certain amount in order to preserve signal quality in a particular application. The movement away from saturation is referred to as the “back-off” of the amplifier, and is a measure of how much the output power must be reduced by in order to achieve the desired characteristics of the output signal. The degree of back-off can thus vary with particular applications, depending on the desired output signal quality.
The disadvantage of this configuration is that the amplifier is not operating at optimum efficiency, the “off-the shelf” configuration of the amplifier being for optimisation at the saturation point of the amplifier as described above. This can be particularly disadvantageous in, for example, satellite communications systems where DC power consumption, dissipation of heat and signal power are crucial design considerations, and sub-optimal operation can be particularly costly.
In multiport amplifier (MPA) systems, for example, which potentially comprise of a large number of amplifier paths between an input network and an output network, and where each amplifier is operating with a large number of signals, it is particularly important to avoid unwanted intermodulation interference but to back off each of the amplifiers from saturation will significantly compromise the overall efficiency of the MPA, reducing performance and suitability for use in space. Similarly, for direct radiating phased array type antennas and phased array fed reflector type antennas, where multiple amplifiers are used in order to amplify signals for each antenna element, all of the amplifiers will be running at less than optimum efficiency.
The present invention aims to improve the DC power efficiency performance of such amplifier systems whilst maintaining the overall signal linearity at the chosen operating point.